1. Field of the Invention
The present invention relates to an optical device which mounts a semiconductor light emitting element on a substrate including a light waveguide path, a method of manufacturing the same, an optical module, and an optical transmission system.
2. Related Art of the Invention
FIGS. 15(a) and (b) show a conventional optical transmission module 160 mounting a semiconductor light emitting element 152 on a glass substrate 151 including a waveguide path 161. When the semiconductor light emitting element 152 mounted on the glass substrate 151 is an edge emitting type element and a high speed operation is required, it is arranged on the glass substrate 151 so that a surface which opposes an active layer junction surface (emitting portion) 155 of the semiconductor light emitting element 152 and has the smallest distance from the active layer junction surface 155 (i.e., active layer proximity surface 154) of the element faces upper. Via holes 162 are then formed in the glass substrate 151, heat generated in the semiconductor light emitting element 152 is radiated to a metal heat sink 153 arranged on the lower side of the glass substrate 151 through a conductive paste 163 which fills the via holes 162, and a common ground potential is simultaneously obtained.
In this case, in order to increase a heat radiation effect from the semiconductor light emitting element 152 to the heat sink 153, it will be desirable to reduce thermal resistance (i.e., increase thermal conductivity) of a thermal path to the heat sink 153 from the semiconductor light emitting element 152. That is, considering the via holes 162 and the conductive paste 163 which fills them to be a thermal resistance element, it will be desirable to decrease the glass substrate 151 in thickness where the via holes 162 are formed (i.e., shorten the thermal resistance element in length), or to increase the diameter of the via holes 162 in width (i.e., increase the diameter of the thermal resistance material in width). However, there is a limit to decreasing the glass substrate 151 in thickness since the integrity of the glass substrate 151 of its own would decrease in strength. In addition, there is also a limit to increasing the diameter of the via holes 162 in width since the glass substrate 151 would decrease in strength. Thus, the heat generated from the semiconductor light emitting element 152 has been radiated to the heat sink 153 through a plurality of the via holes 162 not by increasing the diameter of the via holes 162 in width but by forming a number of the via holes 162 on the glass substrate 151 as illustrated in FIG. 15 (See Japanese Patent gazette Laid-Open No. 2002-131593. The disclosure of the above document is incorporated herein by reference in its entirety.)
However, forming a number of the via holes 162 with a high aspect ratio in the glass substrate 151 as described above has been a factor in increasing the cost because of manufacturing difficulty and an increase of manufacturing process. In addition, there have been cases where even when a number of the via holes 162 in the glass substrate 151 have been formed, the thermal resistance to the heat sink 153 from the semiconductor light emitting element 152 has not been able to be reduced, so that the heat radiation to the heat sink 153 from the semiconductor light emitting element 152 has not been enough. In that case, heat radiation from other heat radiation paths (e.g., heat radiation from peripheral air) must be depended on, in addition, the speed of heat transfer from the semiconductor light emitting element 152 to the heat sink 153 is not high enough, so it is necessary to increase the heat sink 153 by one semiconductor light emitting element 152 in size, therefore, the heat sink 153 having large area has been needed in order to manufacture one optical device, further, increasing packaging density on the glass substrate 151 corresponding thereto has not been achieved. In addition, forming a number of the via holes 162 in the glass substrate 151 reduces the glass substrate 151 in strength, resulting in upsizing the optical device and increasing the cost corresponding thereto.